Most crown prosthetic appliance formed of metals are produced by the lost wax process with high casting accuracy.
The investment material to be used for casting is required to possess the following properties:
(1) It has sufficient heat resistance. PA0 (2) It expands so uniformly that the shrinkage on casting of a metal can be made up for. PA0 (3) It has good reproducibility of the surface state of a wax pattern serving as a prototype. PA0 (4) It has good air permeability. PA0 (5) It shows favorable mold release characteristics with respect to cast products without undergoing any baking or chemical reaction. PA0 (6) It has good flowability in a slurried state (before setting).
Currently, use is mainly made of investment materials in which quartz or cristobalite is used as the refractory material. However, it is hard to say that such investment materials sufficiently possess the aforesaid required properties. Many type of alloys for example gold alloy, silver alloy, gold-silver-palladium alloy, silver-palladium alloy, palladium alloy, nickel-chromium alloy, cobalt-chromium alloy, etc., are primarily used as the metals for dental casting. When such alloys are cast in a casting mold, however, there arises shrinkage on casting, or a decrease in volume, which is caused by thermal shrinkage of a melt, a volume change on solidification, thermal shrinkage taking place during cooling to room temperature following solidification, so that the resulting cast product is smaller in size than the prototype. In order to obtain the cast product of the same size as the prototype, it is thus required to give an investment a coefficient of expansion corresponding to the degree of shrinkage on casting of an alloy, thereby to expand the casting mold. The means for expansion of the casting mold to this end now include a combination of the thermal expansion of investments with the setting expansion (esp. the hygroscopic expansion) thereof.
Referring first to thermal expansion, quartz and cristobalite show thermal expansion due to phase transformation at 500.degree.-600.degree. C. and 200.degree.-300.degree. C., respectively. However, such thermal expansion is insufficient to make up for the shrinkage on casting of a metal. For that reason, the setting expansion (esp. the hygroscopic expansion) is further used to expand the investment. However, the setting expansion (esp. the hygroscopic expansion) of the investment becomes partly uneven, so that the wax pattern may possibly deform, thus offering an accuracy problem.
In order to improve the reproducibility of the surface state of the wax pattern, it is required to finely divide refractory particles. However, this is accompanied by a decrease in the air permeability of the investment, thus leading to the incidence of casting deficiencies such as misrum, or resulting in a decrease in the flowability of the investment in a slurried state, which, in turn, results in deterioration of workability during the investing of wax patterns. It is thus impossible to pulverize such particles to no more than any specific particle size.